<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" value="手机" style="color: #cccccc">
<script>
  var input=document.querySelector("input");
  input.onfocus=function () {
   if(input.value==='手机'){
     input.value='';
   }
   input.style.color='#333';
  }
  input.onblur=function () {
    if(input.value===''){
      input.value='手机';
    }
    input.style.color='#999';
  }
</script>
</body>
</html>
